1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic process.
2. Related Background Art
FIG. 13 shows one example of a conventional electrophotographic color image forming apparatus. The color image forming apparatus of this example comprises a drum-shaped electrophotographic photosensitive member, which serves as an image bearing member, or in other words a photosensitive drum 3. This photosensitive drum 3 is driven in the direction shown by the arrow by driving means (not shown) and is uniformly charged, for example negatively, by a primary charger 4. Next, a laser beam in accordance with a magenta image pattern from an exposure device 5 is irradiated on the photosensitive drum 3 and an electrostatic latent image is formed on the photosensitive drum 3.
A rotary developing device 6 is placed opposite the photosensitive drum 3. This rotary developing device 6 comprises a rotary supporting member 60 and a plurality of developing units detachably mounted on the rotary supporting member 60. In this example there are four developing units 6a, 6b, 6c, and 6d.
When the photosensitive drum 3 with the above electrostatic latent image formed on it rotates in the direction shown by the arrow, the developing unit 6a containing magenta toner having a negative polarity among the developing units supported by the rotary supporting member 60 rotates around to be positioned opposite the photosensitive drum 3 so that the electrostatic latent image on the photosensitive drum 3 is visualized developed through so-called reverse development by the selected developing unit 6a, or in other words, is made into a toner image.
A second image bearing member 2 is positioned below the photosensitive drum 3. In this example, the second image bearing member is an intermediate transfer belt 2, which serves as an intermediate transfer member, stretched around rollers 21, 23, and 24, and rotated in the direction shown by the arrow at approximately the same speed as the photosensitive drum 3. The toner image formed on the photosensitive drum 3 undergoes a primary transfer onto the outside peripheral surface of the intermediate transfer belt 2 through a primary transfer bias applied to a primary transfer roller 22 at the primary transfer portion (the primary transfer position).
A plurality of color toner images can be superimposed and transferred onto the intermediate transfer belt 2 by performing the above process with cyan, yellow, and black.
Next, a transfer material 11 is fed from inside a transfer material cassette 13 by a pick-up roller 12 with a designated timing. Simultaneously, a secondary transfer bias is applied to a secondary transfer roller 7, which serves as a secondary transfer device, and the toner image is collectively transferred from the intermediate transfer belt 2 to the transfer material 11. The transfer material 11 is conveyed to a fixing device 10 by a conveying belt 9 and a color image is obtained by fusing and fixing the transferred toner image.
Residual toner (residual toner after secondary transfer) on the intermediate transfer belt 2 after completion of secondary transfer is charged, in the present example positively, to the opposite polarity by a charging device 200 such as a contact charger, for example, which serves as an intermediate transfer belt cleaning device. This residual toner after transfer, which has been charged to the opposite polarity, is conveyed to the primary transfer portion again by moving the intermediate transfer belt 2 and counter transferred onto the photosensitive drum 3, which is the counter-electrode, by a primary transfer bias applied to the primary transfer roller 22. The counter transfer toner (residual toner after secondary transfer) and the residual toner after primary transfer on the photosensitive drum 3 are cleaned by a cleaning device 8.
FIG. 14 shows another example of a conventional electrophotographic color image forming apparatus. This color image forming apparatus is called a tandem system or an inline system apparatus and, as opposed to the color image forming apparatus shown in FIG. 13, it has a photosensitive drum 3 for each color (3a, 3b, 3c, and 3d) and a developing unit 6a, 6b, 6c, and 6d corresponding to each of the drums.
Each photosensitive drum 3a, 3b, 3c, and 3d is placed serially on the intermediate transfer belt 2 and uses a method for forming a color image in unison per rotation of the intermediate transfer belt 2. As a result, faster printing is possible.
With this system, it is also possible to clean the intermediate transfer belt with the contact charger 200, which is the above intermediate transfer belt cleaning device. In this case, a cleaning device 8a built collateral to the first color photosensitive drum 3a collects the residual toner after secondary transfer.
Also, the intermediate transfer belt cleaning device 200 in the tandem system, as opposed to in the one-drum system described above, usually abuts against the intermediate transfer belt 2, which serves as an intermediate transfer member.
The mechanism of intermediate transfer belt cleaning is now described in further detail. The mechanism of intermediate transfer belt cleaning is identical in the image forming apparatus of FIG. 13 and the color image forming apparatus of FIG. 14. Therefore, it will be described in relation to the image forming apparatus of FIG. 13.
When the toner is transferred from the intermediate transfer belt 2 to the transfer material 11, it is subjected to a strong magnetic field of the opposite polarity to the toner and much of the toner remains on the intermediate transfer belt 2, charged to the opposite polarity (in this example, positive) to its regular charging polarity (in this example, negative) as residual toner after second transfer. However, this does not mean that all of the toner is reversed to a positive polarity. Neutralized toner without a charge and toner maintaining its negative polarity also exists.
Thus, a contact charger is installed as the intermediate transfer belt cleaning device 200 immediately after the secondary transfer position and a voltage in which an alternating-current component is superimposed on a direct current component is applied as the intermediate transfer belt cleaning bias. The residual toner after secondary transfer repeats a reciprocating movement due to the alternating-current component and is charged more uniformly to a positive polarity.
The residual toner after secondary transfer, which has been uniformly charged to a positive polarity, is counter-transferred onto the photosensitive drum 3 at the primary transfer nip and collected by the cleaning device 8 on the photosensitive drum 3.
Even during continuous printing, the charge of the oppositely charged residual toner after secondary transfer on the intermediate transfer belt 2 and the regular charge of the toner transferred from the photosensitive drum 3 at primary transfer are not offset by contact of a short term. Accordingly, it is possible to transfer each type of toner at the primary transfer portion: residual toner after secondary transfer is transferred onto the photosensitive drum 3, and regular toner on the photosensitive drum 3 is transferred onto the intermediate transfer belt 2. In this way, residual toner after secondary transfer is not transferred onto the transfer material 11 during the next print and an accurate image is output.
If the above intermediate transfer belt cleaning device 200 is used, a waste toner container for collecting residual toner after transfer on the intermediate transfer belt 2 can be used in conjunction with the cleaning device 8 on the photosensitive drum 3, enabling the device to be made more compact and improving maintainability.
For the intermediate transfer belt 2 used in the above color image forming apparatus, there is a non-charge type comprising a material with a value of volume resistivity of approximately 10.sup.5 .OMEGA.cm to 10.sup.8 .OMEGA.cm, which is itself difficult to charge by toner delivery or by the charge of the primary transfer and secondary transfer biases, and a charge type comprising a material with a value of volume resistivity of approximately 10.sup.9 .OMEGA.cm to 10.sup.16 .OMEGA.cm, which is itself easy to charge.
Many constructions of the charge type intermediate transfer belt 2 have been designed. For example, a belt has been designed as shown in FIGS. 7 and 8 in which the intermediate transfer belt 2 has two-ply or three-ply construction, wherein the value of volume resistivity of the surface layer 2A of the belt in FIG. 7 and the intermediate layer 2B of the belt in FIG. 8 (approximately 5 to 50 .mu.m thick) is approximately 10.sup.10 .OMEGA.cm to 10.sup.16 .OMEGA.cm, the base layer 2C is of a material with a value of volume resistivity of 10.sup.3 .OMEGA.cm to 10.sup.8 .OMEGA.cm, and the belt as a whole is 10.sup.9 .OMEGA.cm to 10.sup.16 .OMEGA.cm.
FIG. 9 and FIG. 10 show the state of the toner on the above intermediate transfer belts 2. As shown in FIG. 10, when a charge type structure is used as the intermediate transfer belt, as compared to the use of a non-charge type as shown in FIG. 9, scattering of the toner, which becomes a problem when layering colors on the intermediate transfer belt 2, can be reduced because the absolute value of the electric potential of the superimposed toner image is lower than that of the non-image portion, and image quality is improved.
No matter which type of intermediate transfer belt 2 above is used, the above-described intermediate transfer belt cleaning can be performed. However, when a charge type intermediate transfer belt is used, it is also possible to eliminate the excess charge on the intermediate transfer belt 2 with the intermediate transfer belt cleaning device 1.
For the intermediate transfer belt cleaning bias including such an elimination ability, a rectangular waveform with high charging capability and a large high-frequency component is used as the AC bias and a deflected waveform such as that shown in FIG. 11 is used in order to coexist with the charging polarity of the residual toner after secondary transfer. The convergent electric potential when such waveforms are used is not the integrated average value of the waveforms, but the electric potential in the exact middle between Vmax and Vmin, or Vmin+(Vmax-Vmin)/2.
To give one example of an intermediate transfer belt cleaning bias, if an alternating current bias wherein the frequency is 3 kHz, the alternating current component is (Vp-p)=2.4 kVpp, the +Duty is 80%, and the direct current component is (Vdc)=+720 V to 920 V is applied, the charging electric potential is between 0 V and +200 V.
As mentioned above, the intermediate transfer belt cleaning device for a charge type intermediate transfer belt fills two roles: unifying the polarity of the residual toner after secondary transfer to a positive polarity (the opposite polarity of the regular charging polarity) and to initialize the surface electric potential of the intermediate transfer belt charged positively by the secondary transfer bias. Accordingly, the intermediate transfer belt cleaning device must both fulfill the uniform charging of the toner and the uniform charging of the belt surface.
However, when the bias applied to the above intermediate transfer belt cleaning device is turned off, a non-uniform charging state due to the reasons explained below exists and an uneven charge is generated on the intermediate transfer belt.
An uneven charge on the intermediate transfer belt will cause unevenness in the primary transfer of the image to follow as well. In particular, a uniform half-tone image will have blank areas due to a decrease in transfer efficiency or will deviate from the optimum transfer electric field range and the half-tone image will scatter with an uneven density degrading the quality of the image markedly. These inconveniences result when the intermediate transfer belt cleaning device is a low-cost contact charger that generates a low amount of ozone, such as a roller charger for example.
The mechanism causing an uneven charge on the intermediate transfer belt originates in the method of discharge within the discharge range of the contact charger when the cleaning bias is turned off. In order for the cleaning bias to charge the intermediate transfer belt after secondary transfer uniformly, it is necessary to eliminate for one full revolution from the starting point of the intermediate transfer belt, but by discharging after this elimination is complete, an uneven electric potential results as described below.
FIGS. 12A to 12C show the waveform when the cleaning bias supplied to the intermediate transfer belt cleaning device is turned off and the type of uneven electric potential that results on the intermediate transfer belt thereafter.
As can be understood from the Figures, when the cleaning bias is turned off, the electric potential of the intermediate transfer belt at that time is determined in accordance with the position at which the AC bias applied to the roller charger is cut off, or in other words, according to where the last electric potential was. If the difference in electric potential between this last electric potential and the surface electric potential of the intermediate transfer belt within the discharge nip is higher than the voltage at commencement of discharge, a final discharge is performed and the electric potential becomes the last electric potential minus the voltage at commencement of discharge (approximately 500 V). Accordingly, it is worst to turn off the bias at the apex of the AC bias and when using a rectangular wave the above problem is striking.
In order to solve this problem, there are methods in which the cleaning bias is turned off on the non-image portion on the intermediate transfer belt, but because the non-image portion will not necessarily be on the photosensitive drum corresponding to the non-image portion, an uneven electric potential on the intermediate transfer belt will be traced on the photosensitive drum at a primary transfer portion as a transfer memory, the uneven electric potential on the photosensitive drum can not be erased during primary charging, and will appear again on the image as a primary charging malfunction.